This application is based on claims priority under 35 U.S.C. xc2xa7 119 with respect to Japanese Patent Application No. 2000-325012 filed on Oct. 25, 2000 and Japanese Patent Application No. 2000-361363 filed on Nov. 28, 2000, the entire content of both of which is incorporated herein by reference.
This invention generally relates to an automatic transmission. More particularly, the present invention pertains to an automatic transmission for a vehicle that supplies six forward shift stages.
An automatic transmission provided with six forward shift stages is disclosed in Japanese Patent Laid-Open Application No. 6(1994)-323381 and Japanese Patent Laid-Open Application No. 2000-234655. The automatic transmission disclosed in Japanese Patent Laid-Open Application No. 6(1994)-323381 is designed to supply gear ratios (1.90, 1.914) via a second shift stage which are smaller than a gear ratio 2.
The automatic transmission described in Japanese Patent Laid-Open Application No. 2000-234655 is designed to supply a gear ratio of 2.7 via a second shift stage. Accordingly, a gear ratio step between a first shift stage and the second shift stage may not be sufficiently large as expected or desired and the gear ratio step between the second shift stage and a third shift stage may be larger than expected or desired. Therefore, optimal gear ratio steps may not be easily generated between any of the shift stages.
It is thus seen that known automatic transmissions are susceptible of certain improvements with respect to generating optimal gear ratio steps between any of the shift stages in the automatic transmission.
An automatic transmission for a vehicle includes an input shaft connected to an output shaft of an engine, an output shaft connected to an axle shaft via a differential gear, and a torque converter for transmitting driving torque from the engine to the automatic transmission. A planetary gear system included in the automatic transmission is provided with first, second, third and fourth planetary gear sets. The first gear set has a first sun gear, a first ring gear, a pair of first pinion gears meshing with the first sun gear and the first ring gear, and a first planetary carrier rotatably supporting the first pinion gears. The first sun gear is connected to the input shaft. The second, third and fourth gear sets respectively have second, third and fourth sun gears, second, third and fourth ring gears, second, third and fourth single pinion gears meshing with each sun gear and ring gear, and second, third and fourth planetary carriers rotatably supporting the second, third and fourth pinion gears. The first and second ring gears and the third sun gear are rotatably connected by a first rotational element. The second, third and fourth planetary carriers are rotatably connected by a second rotational element. The third ring gear, the fourth sun gear and the output shaft are rotatably connected by a third rotational element. The second sun gear and the input shaft are rotatably connected by a fourth rotational element. A first clutch selectively connects and releases the second ring gear and the third sun gear, and a second clutch selectively connects and releases the second rotational element and the input shaft. A first brake selectively holds the first rotational element against rotation and releases the first rotational element, a second brake selectively holds the fourth ring gear against rotation and releases the fourth ring gear, and a third brake selectively holds the first planetary carrier against rotation and releases the first planetary carrier.
In an alternative construction, the first clutch selectively connects and releases the second and third planetary carriers. Another alternative involves the first planetary carrier being connected to the input shaft, and the third brake selectively holding the first sun gear against rotation and releasing the first sun gear. Another variation involves the first planetary carrier being connected to the input shaft, the first clutch selectively connecting and releasing the second and third planetary carriers, and the third brake selectively holding the first sun gear against rotation and releasing the first sun gear.
According to another aspect of the invention, an automatic transmission for a vehicle includes an input shaft connected to an output shaft of an engine, an output shaft connected to an axle shaft via a differential gear, a torque converter for transmitting driving torque from the engine to the automatic transmission, and a planetary gear system. The planetary gear system includes first, second, third and fourth planetary gear sets, with the first gear set having a first sun gear, a first ring gear, a pair of first pinion gears meshing with the first sun gear and the first ring gear, and a first planetary carrier rotatably supporting the first pinion gears. The first sun gear is connected to the input shaft, and the second, third and fourth gear sets respectively having second, third and fourth sun gears, second, third and fourth ring gears, second, third and fourth single pinion gears meshing with each sun gear and ring gear, and second, third and fourth planetary carriers rotatably supporting the second, third and fourth pinion gears. A first rotational element rotatably connects the first ring gear and the second sun gear, a second rotational element rotatably connects the second and fourth planetary carriers and the third ring gear, a third rotational element rotatably connects the second ring gear, the third planetary carrier and the fourth sun gear to the output shaft, a fourth rotational element is rotatably connected to the third sun gear, a first clutch selectively connects and releases the fourth rotational element and the input shaft, a second clutch selectively connects and releases the second rotational element and the input shaft, a first brake selectively holds the first rotational element against rotation and releases the first rotational element, a second brake selectively holds the fourth ring gear against rotation and releases the fourth ring gear, and a third brake selectively holds the first planetary carrier against rotation and releases the first planetary carrier.
In an alternative to this construction of the automatic transmission, the first clutch selectively connects and releases the second planetary carrier and the third ring gear, and the third sun gear and the input shaft are rotatably connected by the fourth rotational element. In another alternative, the first planetary carrier is designed to be constantly connected to the input shaft, and the third brake selectively holds the first sun gear against rotation and releases the first sun gear. According to another alternative, the first planetary carrier is designed to be constantly connected to the input shaft, the first clutch selectively connects and releases the second planetary carrier and the third ring gear, the third sun gear is rotatably connected to the input shaft by the fourth rotational element, and the third brake selectively holds the first sun gear against rotation and releases the first sun gear. In a further alternative, the second planetary carrier and the third ring gear are rotatably connected by the second rotational element, the second ring gear and the third and fourth planetary carriers are rotatably connected to the output shaft by the third rotational element, and the third and fourth sun gears are rotatably connected by the fourth rotational element. This construction can be varied so that the first planetary carrier is designed to be constantly connected to the input shaft, the second ring gear and the third and fourth carriers are rotatably connected to the output shaft by the third rotational element, the fourth sun gear is rotatably connected to the fourth rotational element, the second planetary carrier and the third ring gear are rotatably connected by the second rotational element, and the third brake selectively holds the first sun gear against rotation and releases the first sun gear.
According to another aspect of the invention, an automatic transmission for a vehicle includes an input shaft connected to an output shaft of an engine, an output shaft connected to an axle shaft via a differential gear, a torque converter for transmitting driving torque from the engine to the automatic transmission, and a planetary gear system having first, second and third planetary gear sets, with the first gear set having a first sun gear, a first ring gear, a pair of first pinion gears meshing with the first sun gear and the first ring gear, and a first planetary carrier rotatably supporting the first pinion gears. The second and third gear sets respectively have second and third sun gears, second and third ring gears, second and third single pinion gears meshing with each sun gear and ring gear, and second and third planetary carriers rotatably supporting the second and third pinion gears. A first rotational element rotatably connects the first ring gear and the second sun gear, and connects the second and third planetary gear sets, a second rotational element rotatably connects to the second planetary carrier, a third rotational element rotatably connects the third planetary carrier and the output shaft, and a fourth rotational element is connected to the third sun gear. A planetary gear unit includes the second and third planetary gear sets and the first, second, third and fourth rotational elements. The planetary gear unit connects the second planetary carrier and the third ring gear, and connects the second ring gear and the third planetary carrier. A first clutch selectively connects and releases the input shaft and the second rotational element, a second clutch selectively connects and releases the input shaft and the first sun gear, a third clutch selectively connects and releases the first sun gear and the first planetary carrier, a first brake selectively holds the first planetary carrier against rotation and releases the first planetary carrier, a second brake selectively holds the second rotational element against rotation and releases the second rotational element, and a third brake selectively holds the fourth rotational element against rotation and releases the fourth rotational element.
In an alternative to this construction, the planetary gear unit connects the second ring gear and the third planetary carrier, and further connects the second and third sun gears. The first rotational element is rotatably connected to the third ring gear, the second rotational element is rotatably connected to the second ring gear, the third rotational element is rotatably connected to the second planetary carrier, and the fourth rotational element is rotatably connected to the third sun gear. According to another alternative, the planetary gear unit rotatably connects the second and third planetary carriers, and rotatably connects the second ring gear and the third sun gear. In addition, the first rotational element is rotatably connected to the second ring gear, the second rotational element is rotatably connected to the second planetary carrier, the third rotational element is rotatably connected to the third ring gear, and the fourth rotational element is rotatably connected to the second sun gear.